Handedness is a prominent manifestation of asymmetry in neural organization that has previously been relegated as either an artifact of lateralization in non-motor systems, or as the simple effect of practiced patterns of hand use. The studies in the initial phase of this award gave rise to a new hypothesis, that handedness reflects a functional optimization process through which control of limb position and trajectory have become specialized. This theory diverges from other biological descriptions of motor lateralization in defining the control mechanisms that underlie handedness, and in proposing advantages for the non- dominant system. However, it remains speculative due to limited evidence for specialization of the non- dominant system. Our first aim tests whether the non-dominant system displays advantages in controlling limb position and posture, and explores the mechanisms that might underlie such advantages. The proposition that lateralization reflects an optimization process leads to two predictions about subjects with varied handedness: First, left handers should show specializations that mirror image those of right handers, and second, mixed handers should exhibit relative deficiencies in motor performance because they have not developed lateral specializations. These predictions are examined in our second and third aims, which serve as critical tests of our model. Our fourth aim examines the implications of changes in lateralization that might occur with aging. It has been well established that lateralization in certain cognitive functions decreases with age. We hypothesize that age-dependent reductions in motor lateralization might give rise to coordination deficits in elderly individuals, a phenomenon that remains incompletely understood. To test this idea, we will examine motor lateralization in three age groups that characterize the adult lifespan. The proposed research has strong implications for neural lateralization theory, as well as for human health. The idea that lateralization reflects specialization of each hemisphere for different functions might explain the role of ipsilateral hemisphere in controlling unilateral arm movements, an effect that is well documented but is not well understood. More importantly, unilateral hemisphere damage due to stroke produces ipsilesional deficits that limit functional performance and that have been shown to reflect motor lateralization. The proposed work should help elucidate the mechanisms that underlie such deficits.